An important class of hematopoietic and immunological interactions occurs between the stem and progenitor cells and the stromal cells of blood forming tissues. Hematopoietic progenitor cells and more recently various types of lymphohematopoietic stromal cells can be grown and cloned in culture. The interaction between progenitor cells and stromal cells can thus be investigated by combining these cells in culture. Unfortunately, a major problem is the identification and location in vivo of cells equivalent to the stromal cell types observed in such cultures. The objective of this project is the development and evaluation of reagents to identify and manipulate murine lymphohematopoietic stromal cells. The starting material will be either trypsinized or detergent treated adherent cell layers obtained from Dexter-type liquid cultures prepared from mouse bone marrow and fetal liver, low temperature (24 C) treated organ cultures of fetal thymus, and Percoll gradient separated populations from suspensions of whole bone marrow, fetal liver, thymus, lymph nodes and skin. Our primary emphasis will be to generate monoclonal antibodies to surface determinants of such stromal cells employing the hydridoma technique of Kohler and Milstein. The cell preparations will be used to immunize DA rats. Hybridomas will be generated using a myeloma of BALB/c mouse background. Cloned lymphohematopoietic stromal cell populations, Dexter-type "monolayers" and whole tissues will be employed to assess the specificity of these reagents using fluorescence, immune electron microscopic and cytotoxicity techniques. These reagents will then be employed with a cell sorter to isolate, manipulate and clone the various stromal cells so as to permit a further evaluation of their properties. Until we can demonstrate that it is possible to develop appropriate monoclonal antibodies, we will also raise antisera to these cellular constituents by conventional techniques employing rabbits or goats. These antisera will be purified and repeatedly absorbed in order to isolate selective, highly reactive reagents. Such reagents, especially those of monoclonal origin, will be useful in the identification, manipulation and investigation of the role of lymphohematopoietic stromal cells in progenitor cell differentiation in normal and diseased states. Ultimately, they may have diagnostic usefulness.